1. Field of the Invention
This invention relates to a gas discharge lamp, having a straight tubular radiation body.
2. Prior Art
Due to their good radiation capacity, gas discharge lamps are often used for irradiation purposes. For certain fields of application it is necessary that the irradiation power of the lamp be constant or almost constant along its entire length. When the lamp is used in copying devices, the clearness and the contrast of the copied letters over the width of the paper sheet depends essentially on a constant irradiation power across the width of the document being copied. Sometimes an over-exposure of the edges is desired, in order to compensate for the marginal drop caused by the optical system of the copying device.
Lamps of this type are also used in devices for the automatic testing of moving material for holes, cracks or other faults, by monitoring the change of the light transmitting quality of the material. The material is linearly exposed to a constant irradiation power across its width. If there are defective spots in the material width, then there will appear changes of the light intensity on that side of the material disposed away from the light source, which will be detected by photo-electric receivers.
In order to generate a linear area with an almost constant irradiation power, oblong light sources, e.g. fluorescent tubes, have been used in conjunction with cylindrical lenses. The irradiation power of the conventional fluorescent tubes charged with neon, argon, krypton or nitrogen, is approximately constant only in the middle portion thereof. The irradiation power continually decreases in a direction toward the ends of the fluorescent tube. The radiant intensity of the lamp itself is constant to the ends, but the irradiation power measured in a plane parallel to the axis of the lamp decreases toward the end portions of the lamp. Therefore, with the conventional fluorescent tubes one can generate oblong areas with almost constant irradiation power on objects only by using corresponding diaphragms. Thus it can be seen that a considerable portion of the radiation emitted by the light source will not be utilized, resulting in a smaller luminous efficiency than is desireable.
Also, in many instances incandescent lamps are used wherein the lamps have a tubular lamp body, in which is arranged a helix running along the lamp axis. In order to intensify the irradiation power of these tubular incandescent lamps in the vicinity of the connecting ends, it is possible to provide the helix wire with more turns per unit length at both ends than in the middle portion of the incandescent lamp. Thus a greater portion of the generated radiation is utilized. However, due to such a complicated construction, the incadescent lamps are difficult and costly to manufacture. It is further known in the art to contiguously arrange individual lamps along a straight line, in order to obtain a rather uniform irradiation power (German Registered Article Specification No. 6,944,880).